The Calcarenite di Gravina Formation in Matera (southern Italy): New insights for coarse-grained, large-scale, cross-beded bodies encased in offshore deposits

摘要

Excellent outcrops of the upper Pliocene-lower Pleistocene Cal- carenite di Gravina around Matera (southern Italy) provide contin- uous exposure of coarse-grained, clastic basin-margin, shoreline to offshore facies. Among these facies, the most conspicuous and vol- umetrically important are the transition-slope deposits that form large-scale, high-angle, cross-bedded lithosomes. These are laterally extensive, parallel with the paleoshoreline, and show seaward pro- gradation. We interpret them to represent avalanches of sediment swept out onto a depositional slope, below wave base, from the shoreface zone by storm waves and wind-driven currents. Three types of building blocks are recognized based on bedding patterns and facies architecture: embryonic parasequences, mature parasequences, and simple sequences. Parasequences formed during stillstands of sea level and simple sequences during high-frequency cycles of relative change of sea level. These building blocks are stacked in a backstepping configuration and onlap onto Cretaceous limestone substrate. Backstepping is believed to be due to a tecton- ically forced transgression that is punctuated by high-frequency cy- cles of sea level. Modem analogs for these building blocks are the Holocene pro- grading prisms detected in high-resolution seismic lines of the Med- iterranean shelves. The reflection patterns of these seismic units resemble the bedding architecture of the Matera simple sequences and parasequences and show similarities of shape, size, position, and orientation of coastal setting, and direction ofprogradation. Com- parison of the Holocene prisms and the Pliocene-Pleistocene ac- cretional units in Matera indicates that they may represent the same genetic process: progradation of clastic prisms below the wave base level. The Matera accretional units also show similarities with other examples of laterally extensive, large-scale, cross-bedded sand bodies encased in offshore deposits, such as some sandbanks. Differentiating between these laterally extensive and seaward-prograding cross-bed- ded sand bodies and some coarse-grained Gilbert-type deltas is difficult, however, if interpretations are based only on two-dimensional (2-D) outcrops in dip section. Without high-resolution data it is also difficult to dis- tinguish between transition-slope and prograding shoreface lithosomes. This difficulty may be acute where only seismic or well-Iog data are available; how- ever, certain other architectural characteristics, such as stacking and preservation of facies belts and position of bounding surfaces, as well as differences in associated sedimentary structures and fossil content, may be used for interpretation. The Matera example provides a mechanism for emplacement in offshore settings of elongate and strandline-parallel sand and gravel deposits that pro- grade seaward and that preserve a coarsening-upward internal succession. This article offers an interpretation for other ancient examples oflarge-scale, cross-bedded lithosomes encased in offshore deposits.